This invention relates generally to valves of the butterfly type and, more particularly, to a double-seal valve structure in which a butterfly valve provides one of the seals and the other seal is provided by a rupturable disk arranged to be ruptured upon actuation of the butterfly valve. This valve structure is particularly useful in carrying out the method for control of oil leakage from a damaged tanker described in applicant's copending application Ser. No. 07/451,103, filed Dec. 15, 1989.
Briefly, the method described in the prior application minimizes the outflow of cargo from a damaged cargo tank of an oil tanker of the "segregated ballast" type by transferring oil out of the upper part of the damaged cargo tank and, at the same time, keeping the reduction of draft at the damaged area to a minimum. This is accomplished by valves and piping installed through selected bulkheads to connect each cargo tank to one or more ballast tanks, one or more of which are opened upon occurrence of damage sufficient to cause cargo tank leakage to allow oil to flow, by gravity, from the upper part of the damaged cargo tank or tanks to one or more ballast tanks which, if the tanker were fully loaded, would be empty. In the described system, each connection between a cargo tank and a ballast tank includes two valves, preferably of the butterfly type, connected in tandem, as required by regulations, to prevent oil contamination of ballast due to possible leakage of a valve in normal operation. Typically, the valves may have a diameter in the range between one foot and four feet, the choice being a trade-off between the cost of the valves and the rate at which it is desired to transfer oil from a damaged tank to one or more ballast tanks, and each valve is equipped with a valve actuator to quickly open or close the valve. Considering that a typical tanker may require at least ten such pairs of valves to effectively control oil leakage by the described method, the installation is obviously very expensive, not only from the standpoint of the cost of the valves themselves but also the cost of their installation.
It is a primary object of present invention to provide a valve arrangement for initiating gravity flow from a cargo tank to a ballast tank which is more effective for the purpose, and less expensive, than the paired butterfly valves of the prior art.
Another object is to provide a valve structure which satisfies regulations that require any connection between a cargo tank and a ballast tank to include two valves in tandem to prevent contamination of ballast, without relying on valve seals.
Still another object is to provide a valve arrangement of the character described which is relatively simple to construct at moderate cost.
Other objects, features and advantages of the invention, and a better understanding of its construction and operation, will be apparent from the following detailed description, taken in conjunction with the accompanying drawings.